This invention relates to the separation of a gas from a mixture of gases. More particularly, this invention is concerned with the utilization of ion-exchange membranes containing selected counter-ions which undergo reversible reaction with a particular gas, thus accomplishing facilitated transport of that gas across the membrane.
Heretofore immobilized liquid membranes have been developed and utilized for the separation of a specific gas from a mixture of gases having diverse or similar characteristics.
U.S. Pat. No. 3,396,510, (Ward III and Robb, assigned to the same assignee as the present application and incorporated by reference), discloses the application of the phenomenon of facilitated transport to liquid membranes to substantially increase the separation factor for gases of such a modified liquid membrane. Facilitated transport is made possible by introducing into the immobilized liquid film a large concentration of at least one select, non-volatile specie, which is soluble in the immobilized liquid and is reversibly reactive with the specific gaseous component to be separated from a mixture of gases, the reaction being productive of a soluble non-volatile specie in large concentration. For example Ward III and Robb describe the use of a membrane comprising an aqueous alkali carbonate solution impregnated in the pores of a porous cellulosic membrane to separate CO.sub.2 from gaseous mixtures by facilitated transport.
In U.S. Pat. No. 3,758,603, (Steigelmann et al., assigned to Standard Oil Company, Chicago, Ill.), there is disclosed a process utilizing facilitated transport for separating various aliphatically-unsaturated hydrocarbons from mixtures wherein the aliphatically-unsaturated hydrocarbon to be separated is essentially in a gaseous or vapor phase when it comes in contact with a liquid barrier having dissolved therein one or more metal ions which form a complex with the unsaturated hydrocarbon to be separated. The liquid barrier is generally in contact with a semipermeable membrane which is essentially impermeable to the liquid. The film membrane may be a cellulose acetate membrane or may be made of olefin polymers such as polyethylene, polypropylene and the like.
The liquid barrier contains sufficient water and soluble metal ions to form a suitable complex with at least one aliphatically-unsaturated hydrocarbon component of the feed gas subjected to the separation procedure. The metal ions readily form the complex upon contact with the feed gas mixture, and, in addition, the complex is converted back to the metal ion and an aliphatically-unsaturated hydrocarbon component of the complex, under the conditions which exist on the discharge or release side of the liquid barrier. The released aliphatically-unsaturated hydrocarbons exit the discharge side of the liquid barrier and are removed from the vicinity of the barrier and its supporting structure by suitable means such as by a sweep gas or through the effect of vacuum. The unsaturated hydrocarbon-metal complex forms and is decomposed in the complex metal ion-containing liquid barrier, and as a result the material passing through the barrier is more concentrated with respect to at least one aliphatically-unsaturated hydrocarbon present in the feed gas mixture.
A process for the separation of aliphatically-unsaturated hydrocarbons from mixtures by the combined use of liquid barrier permeation and metal complexing techniques is also disclosed in U.S. Pat. No. 3,758,605 (Hughes et al., assigned to Standard Oil Company, Chicago, Ill.). The liquid barrier is contained within a hydrophilic film membrane, and this liquid barrier contains complex-forming metal ions in an aqueous solution. The metal ions may be selected from transition metals, nickel, mercuric, cuprous or other metal ions or mixtures of these metal ions with or without other cations. The separation of ethylene from methane and ethane is of particular interest.
A particular drawback of the immobilized liquid, facilitated transport membranes is the difficulty of maintaining the integrity of the liquid membranes in the presence of a transmembrane pressure difference. Additionally, when a humidity gradient of the feed gas is present, such as when a substantial fraction of the feed gas is transported through the membrane, there is a tendency of the solute species to migrate along the membrane to the region of highest relative humidity. Indeed, if a condition exists where the humidity of the gas exceeds 100%, and condensation occurs, the salt solution may be washed from the liquid membrane.
In order to achieve a physically useful immobilized liquid membrane structure, the liquid is generally held in the pores of a porous hydrophilic polymer membrane. Typically, these porous membranes may be cellulosic or other hydrophilic material. The transmembrane pressure differential which liquid membranes can withstand is dictated by the capillary forces holding the liquid within the pores of the membrane. Ion-exchange membranes, which are highly swollen by water and aqueous solutions can also be used as the liquid immobilizing membrane. Here the transmembrane pressure differential limit is dictated by the "swelling pressure", that is the internal pressure of the ion-exchange membrane containing the liquid, which resists the squeezing of the imbibed liquid from the membrane pores. Another method of immobilizing the liquid is to use a supporting membrane which is impermeable to the membrane liquid. This membrane may be a porous hydrophobic membrane or a non-porous, but highly permeable membrane. Often, this type of membrane is used to support a liquid membrane immobilized in a porous hydrophilic membrane.
Although immobilized liquid membranes have been successfully utilized in the laboratory and pilot plant, the above limitations inherent to these membranes limit their practical applicability.
The invention described in this patent circumvents the problems inherent in immobilized liquid facilitated transport membranes. In this invention, an ion-exchange membrane with an appropriate mobile counter-ion is chosen from that group of ions which will reversibly combine with the gas to be transported. If the membrane is maintained in a humid atmosphere by humidification of gases in contact with the membrane, the counter-ions will be mobile, and facilitated transport will take place. Since electroneutrality within the membrane must be maintained, the carrier counter-ion cannot be lost by physical processes such as the imposition of a high transmembrane pressure gradient. Thus, many of the limitations associated with immobilized liquid ion-exchange membranes are eliminated. In the case of the immobilized liquid facilitated transport membranes, the facilitating specie was physically held within the structure of the membrane primarily by surface tension forces acting indirectly on the solution containing the carrier species. In our invention, the facilitating specie is actually a functional element of the polymer comprising the membrane. In this situation the carrier species is directly localized by electrical forces acting on the carrier specie as opposed to surface tension forces retaining the carrier specie.
U.S. Pat. No. 3,780,496, Ward III et al., (assigned to the same assignee as the present application and incorporated by reference), discloses a method for the separation of helium, hydrogen and oxygen from gas mixtures. The process disclosed in the Ward III et al. patent recognizes that sulfonated polyxylylene oxide is an ion exchange material with the unique property of being solvent castable into a film. When this ion exchange material is placed in an aqueous salt solution, the active groups dissociate and the counter-ion becomes mobile and subject to displacement by other cations which may be present in the salt solution. Thus, films of sulfonated polyxylylene oxide may be readily converted to a number of counter-ion forms.
The sulfonated polyxylylene oxide membrane prepared according to the described method of Ward III et al. exhibits optimized gas separation properties when select counter-ion forms of the sulfonated polyxylylene membranes are employed for the separation of different gases.
However, while the Ward III et al. patent teaches the use of sulfonated polyxylene oxide as an ion exchange material used in a solution-diffusion, that is, a non-reactive membrane, it does not teach the use of a mobile counter-ion or a counter-ion which must react reversibly with one of the components of the gas mixture.
The gas permeation rate of the membrane prepared from sulfonated polyxylylene oxide as disclosed in the Ward III et al. patent is governed by the physical solubility of the gas in the membrane and the diffusion coefficient of the dissolved gas. According to Applicants' process, the various counter-ions disclosed in the present invention are used to vary the chemical interactions between the gas and the membrane. However, the reversible reactivity of the counter-ion as a facilitated transport carrier retained in the membrane as a counter-ion balancing the charge of the fixed ions in the membrane is neither taught nor suggested by the Ward III et al. patent or by any other reference available in the prior art.